Methods of this type, in which a processing machine transmits a signal which is proportional to the developed area of film and therefore also to the amount of silver passed into the fixing bath, are well known.
With regard to electrolytic silver recovery for a film processing machine, it is known from DE-PS 1 188 822 that the level of recovery is controlled by switching the electrolysing current on and off according to the quantity of the photographic material passing through the fixing bath.
On-line fixing bath regeneration systems for one to three processing machines, in which the fixing bath fluids are continuously fed via connecting pipes into an electrolytic silver extraction unit, where the silver is removed, and then fed back into the fixing baths, are known. (Brochure of the company H. Stamm KG/1991). The electrolysing current is controlled in the familiar manner by signals which are proportional to the silver concentration level of the fluid mixture formed in the recovery unit which is either measured by sensors placed at that point or determined on the basis of the area of film passing through the fixing baths.
The "Ecosys F 08" on-line apparatus for two processing machines produced by the company AgfaGevaert AG performs fixing bath regeneration in a similar manner.
The disadvantage of these methods is the fact that silver extraction for each individual fixing bath is not performed on the basis of the amount of silver passing into the fixing bath, which is relative to the area of film passed through each bath. Optimum silver extraction of each separate fixing bath according to its silver concentration level at any one time is therefore not possible.
Another disadvantage lies in the fact that if the amount of silver passing into each fixing bath varies significantly, a fixing solution mixture is formed in the silver recovery unit whose overall silver concentration is lower than the concentration in the fixing bath with the higher film throughput. Since equal electrolysing currents will achieve higher silver recovery rates with higher silver concentrations than with lower concentrations, rapid recovery of silver from the fixing solution is not possible with this type of method.